Hemostasis is the entire physiological process of maintaining blood in a fluid state within intact blood vessels and preventing excess blood loss by arresting flow via the formation of a hemostatic plug. Normal hemostasis is maintained by tightly regulated interactions of the blood vessel wall, blood platelets and blood plasma proteins. Under normal conditions there is a delicate balance between the individual components of the hemostatic system. Any disturbances in this hemostatic balance, the hemostatic potential, could result in bleeding or thrombosis, FIG. 1. By “hemostatic potential” we mean the ability to maintain a balance between procoagulant and anticoagulant states, as measured by fibrin polymerization, when coagulation is initiated by a trigger or activator.
A thrombotic tendency (thrombophilia) results from the generation of excess thrombin activity and increased fibrin polymerization and clot formation (hypercoagulability) while a bleeding tendency (hemophilia) results from insufficient thrombin generation and reduced fibrin polymerization and clot formation (hypocoagulability). There is as yet no single laboratory parameter that is increased in all forms of hypercoagulability and decreased in all forms of hypocoagulability. This is in part due to factors other than plasma that play a part in hemostasis. As described above, these other factors include the blood vessel wall and platelets. However, large proportions of the hemostatic disorders are related to defects or deficiencies in the blood proteins that constitute the coagulation system. These proteins are responsible for the stabilization of the platelet plug by the formation of fibrin. Therefore, a global measure of the plasma contribution to coagulation would facilitate the investigation and management of patients with altered hemostasis.
Thrombophilia and haemophilia can be either congenital or acquired. The congenital forms have a genetic basis and are therefore not readily corrected. The acquired forms generally result from environmental changes, often the effect of drugs, and are therefore susceptible to manipulation. For example a normal individual given warfarin develops acquired haemophilia, stopping the warfarin abolishes the condition. A normal individual given high dose estrogen develops acquired thrombophilia, stopping the estrogen abolishes the condition. The fundamental basis of both the congenital (genetic) and acquired (environmental) thrombophilias and haemophilias is a change in either the amount or activity of one or more key components of the coagulation pathway. For example the most commonly recognized hereditary form of thrombophilia is a mutation in the factor V gene which results in the production of a structurally altered factor V protein (Factor V Leiden) that is resistant to enzymatic cleavage by protein C, a critical regulatory component. Classical Haemophilia A is due to a mutation in the factor VIII gene which results in either reduced production of factor VIII, or production of a structurally altered factor VIII protein that does not function correctly. In contrast to the congenital thrombophilias and haemophilias the acquired forms do not result from altered structure but rather alteration of the amount of a key component, typically more than one at a time. For example the thrombophilic effect of oestrogen is due to the composite effects of a rise in factors XI, IX, VIII, II and fibrinogen and a reduction in the anticoagulant protein S. The haemophilic effect of warfarin is due to a reduction in factors II, VII, IX and X. FIG. 2 illustrates the various states of coagulability and lists examples of assays used to assess the degree or presence of an imbalance. There is currently not an assay that can be used to assess both hyper and hypocoagulability simultaneously. This is due in part to the complexity of the coagulation process, the interdependence of the various components and the identification of a means to monitor the hemostatic potential of the entire coagulation system. FIG. 3 presents an overview of the coagulation process. The process can be divided into four dependent phases, (1) the initiation phase, (2) the propagation phase, (3) the amplification phase and (4) the polymerization phase. All of the phases are affected by regulatory and feedback processes referred to as anticoagulant pathways.
Initiation or triggering of coagulation occurs by exposure of tissue factor due to vascular damage, plaque rupture or monocyte expression as a result of inflammation. Trace amounts of FVIIa and tissue factor form the extrinsic Xase complex. This complex enhances the catalytic activity of VIIa towards factors X and IX resulting in the formation of the active enzymes Xa and IXa. Factor Xa generated by the extrinsic Xase complex forms a small amount of thrombin (IIa). The thrombin generated is capable of activating small amounts of the cofactors VIII and V. In vivo, the extrinsic Xase complex is quickly inactivated by Tissue Pathway Factor Inhibitor, TFPI, via the formation of a quaternary complex consisting of TF, VIIa and Xa. Under physiological conditions the extrinsic Xase generates only picomolar amounts of thrombin.
During the propagation phase of coagulation the role of the extrinsic Xase is minimized and Factor Xa is alternatively generated by the complex of the enzymes IXa and its cofactor VIIIa. This enzyme complex is referred to as intrinsic Xase. Formation of the Xa by the intrinsic Xase complex is approximately 50 fold more efficient than the extrinsic Xase. Factor Xa and its activated cofactor, FVa, form a complex on the surface of activated platelets. This is an efficient catalyst for the conversion of prothrombin to thrombin, referred to as the prothrombinase complex. Thrombin formed via the intrinsic Xase complex is capable of amplifying its own production by positive feedback (activation). Thrombin activates Factors VIII and V and Factor XI activation leads to further production of the enzymatic component of intrinsic Xase (Factor IXa). Normal thrombin production is highly regulated and localized. TFPI neutralizes the trigger for thrombin generation. Active proteases (IIa, Xa, IXa) must be inactivated by protease inhibitors to avoid disseminated thrombosis. One of the most significant of these inhibitors is antithrombin III (ATIII). Both thrombin and Xa, and to a lesser extent IXa released from membrane surfaces, are rapidly inhibited by ATIII. Thrombin can also bind non-damaged sub-endothelium via a receptor molecule, Thrombomodulin (TM). The formation of the IIa/TM complex changes the substrate specificity of thrombin from a procoagulant to an anticoagulant. Thrombin bound to TM is a potent activator of Protein C, converting it to the active enzyme Activated Protein C (APC). APC together with its cofactor protein S cleaves activated cofactors FVIIa and FVa yielding their inactive forms, FVIIIi and FVi. Thrombomodulin also accelerates the inactivation of thrombin by ATIII.
The formation of thrombin leads ultimately to cleavage of fibrinogen to form fibrin. During the polymerization phase cross-linking of soluble fibrin strands is mediated by Factor XIIIa, an enzyme generated by thrombin activation. The thrombin-TM complex activates the procarboxypeptidase thrombin activated fibrinolysis inhibitor (TAFI). Thus thrombin plays a role during this phase by both influencing the architecture and stabilization of the fibrin clot. Thrombin is a key enzyme and effector of the coagulation process. Thrombin is both a potent procoagulant and anticoagulant. However, it is thrombin's ability to cleave fibrinogen and its contribution to fibrin polymerization events that are critical to maintaining stasis.
Clot initiation, often referred to as clotting time, occurs at the intersection between the initiation and propagation phases when only approximately 5% of thrombin has been formed. The majority of the thrombin formed is generated after the initiation of fibrin polymerization, thus the rate of fibrin polymerization is a more sensitive indicator of the dynamics of coagulation. Changes in the propagation phase, amplification phase and anticoagulant pathways alter the rate of thrombin generation and the impact of thrombin availability on rate of fibrin polymerization. Recent studies by Cawthern et al. (1998) suggested that measurement of this thrombin is more informative than clotting time in assessing the pathophysiology of hemophilias. However these investigators measured thrombin by looking at the kinetics of formation of the thrombin-antithrombin complex (indictor of thrombin generation) and formation of fibrinopeptide A (indicator of fibrinogen cleavage) and not by measuring the kinetics of fibrin polymerization. Variations in concentration or quality of the fibrinogen or fibrin strands can only be measured as a function of the actual polymerization process. Assays currently used to assess variations in the coagulation process typically can only assess variations in one or two phases. These assays measure events independently and therefore negate or eliminate the ability to detect variations in the other phases or interactions between the various phases.
Assays associated with the assessment of bleeding risk include the Prothrombin Time (PT), Activated Partial Thromboplastin Time (aPTT), Thrombin Time (TT) and Fibrinogen (Fib) assays (FIG. 2). These assays are based on the addition of potent activators of the coagulation process and thus are only abnormal when major defects are present. These assays are not designed to detect the composite effect of multiple minor alterations. For example in the PT test, which utilizes a very high concentration of a tissue extract, called thromboplastin, and calcium are added to citrated plasma. Whole blood is mixed with citrate when the blood sample is taken. The citrate binds the calcium and “anticoagulates” the blood as calcium ions are required for assembly of the tenase and prothrombinase complexes. The blood sample is then centrifuged and the plasma is separated. When calcium is added back, the tenase (or Xase) and prothrombinase complexes can form and thrombin can be generated. The source of tissue factor is the thromboplastin. However, the concentration of tissue factor is extremely high (supraphysiological) and so only the initiation phase of thrombin generation is required. The propagation and amplification phases are bypassed. The prothrombin time is therefore insensitive to many changes in the coagulation pathway and is incapable of detecting hypercoaqulability. Assays based on diluted thromboplastin have been formulated to aid in the diagnosis of patients with antiphospholipid syndrome (APS). In these methods the thromboplastin together with the phospholipids are diluted to enhance the sensitivity of the PT to the presence of antiphospholipid antibodies. The dilute PT clotting time is prolonged in APS due the unavailability of phospholipid surfaces and therefore the assay is phospholipid dependent instead of TF dependent.
Assays associated with the assessment of a hypercoagulable state (FIG. 2) include the Thrombin Anti-Thrombin Complex (TAT), Prothrombin fragment F1.2, PAI 1, APCr and D-dimer. These assays are designed to measure a specific marker or product of the coagulation process. For example, the measurement of elevated levels of D-dimer indicates that the clotting process has been activated. However, there is no way of determining whether the D-dimer was being produced as a product of the normal healing process or if there is an underlying hypercoagulable risk. The hypercoagulable state cannot be globally assessed by a single assay but currently requires a battery of tests. A global assay for the assessment of hemostatic potential would be able to identify an imbalance utilizing a single assay principle that is sensitive to defects, singular or in combination. The assay would also be sensitive to effects of intervention to restore the hemostatic balance.
Recognising the limitations of the screening assays available for hypcoagulable assessment and the battery of assays required for hypercoagulable assessment, others have tried to develop global tests. These tests were designed to be sensitive to the amount of the biological components and their interactions, as well as measure the dynamics of thrombin generation including regulation. The thrombin generation curve was described more than 30 years ago as a measure of the thrombin generating potential of plasma. A modification of the thrombin generation curve has been described with quantification of thrombin with a exogenously added chromogenic substrate. This has been called the endogenous thrombin potential (ETP). The assay assumes that there is a direct correlation between endogenous thrombin potential measured via an exogenously added artificial substrate and the assessment of a hemostatic imbalance. The use of an artificial substrate instead of thrombin's natural substrate, fibrinogen, ignores the effects of variations in fibrinongen concentration and fibrinogen configuration. Thrombin is a cleavage product from the proteolysis of Prothrombin, a serine protease. Thrombin then cleaves fibrinogen, its natural substrate, resulting in soluble fibrin monomers that are crossed linked via FXIIIa to formed crossed linked polymerized clots. Thrombin is a highly regulated molecule that possesses both procoagulant and antithrombotic behavior. Additionally, there are numerous substrates that inactivate thrombin before it can cleave fibrinogen. In addition to not directly measuring the ability to form a clot the ETP assay has several other major limitations. Limitations of the test include:    1. The plasma sample must be defibrinated, typically with a snake venom. Defibrinating snake venoms activate FX and they also cleave the chromogenic substrate used to quantitate thrombin. This can cause a variable over-estimate of the thrombin potential.    2 The plasma sample is considerably diluted in order to prolong the dynamics of thrombin generation. This results in a non-physiological regulation of the thrombin explosion.    3 The technique involves multiple subsampling at specified timepoints. For example, a computer linked pipeting device designed in order to terminate thrombin activity in the subsamples exactly at a specified time. It is possible to perform the assay manually but it is beyond the ability of many technologists and requires considerable skill. The test cannot be automated on standard clinical laboratory coagulometers.    4 The formation of thrombin-α2 macroglobulin complex leads to over-estimation of the thrombin potential. A complex mathematical manipulation of the results to approximate it to the true thrombin potential is therefore required.    5 Does not take into account the rate or ability of thrombin to cleave fibrinogen.
Duchemin et al. described a further modification of the ETP where the protein C pathway is assessed by adding exogenous thrombomodulin. This method was also modified to take into account proteins that modulate anticoagulant activity, including antithrombin III. Like ETP, this modified assay is designed to only measure thrombin generation and not the effects of thrombin, i.e. dynamic clot formation.
Other investigators have attempted to design assays sensitive to the composite of biological components of the coagulation process and their interactions. One such example is described by Kraus (Canadian application 2,252,983). The method is however limited to determining the anticoagulant potential of a sample by adding thrombomodulin and thromboplastin in a coagulation test. In the described method the emphasis is on dilutions of thromboplastin such that thrombin is produced at a rate slow enough to enable sufficient activation of protein C during the measuring time of the coagulation apparatus. A disadvantage of this method is that because it depends on clot time, the amount of thromboplastin is more restrictive and higher concentrations are required to compensate for increases in clotting time when thrombomodulin is added. Because the method described is aimed at assessing anticoagulant potential and not global hemostatic potential the assay is not sensitive to defects in the propagation and amplification phases, the kinetics of clot polymerization or to the interrelationships between the factors responsible for thrombin generation.
The present invention however assesses both the anticoagulant and procoagulant potential of a blood sample. Furthermore, the present invention's sensitivity can be enhanced by using more dilute coagulation activator, more dilute than has previously been used, since the endpoint method is not restricted to clot time but analysis can be conducted for the entire dynamic coagulation process as measured by evaluating kinetic parameters of the optical data profile. Analysis of more than simply clot time can be accomplished even when very weak and unstable clots are formed.
Variations in the amplification and/or propagation phases will reduce or alter the rate of generation of thrombin and thus impact the rate of fibrinogen cleavage and ultimately the rate of fibrin polymerization. Because the present invention can measure the rate of fibrin polymerization throughout the dynamic coagulation process, it measures the clinically important thrombin that is generated after clotting time.
Other prior art (Mann et. al.) assesses coagulation problems by taking a series of independent and indirect measurements. Thrombin generation is measured as a function of TAT complex formation or the use of a chromogenic substrate and the formation of fibrin as measured by the release of FPA. All of the systems and models to date have been designed to understand a discrete process or interaction of the coagulation process and cannot provide an assessment of the overall hemostatic potential. In contrast, the method of the present invention is designed to not only assess the interplay of the coagulation proteins together with synthetic cell surfaces, it is aimed at capturing this in a dynamic measurement that correlates to clinical outcome. The technology and methods described in the present invention can also be modified to introduce components of the fibrinolytic system as well as cells and flow conditions.
Givens et. al. demonstrated that a model which characterizes the process of clot formation and utilizes parameters in addition to clotting time is sensitive to defects in the clotting proteins. Table 1 describes the parameters defined by Givens et al. and FIGS. 4 and 5 illustrate how those parameters are determined and how they relate to fibrin polymerization for the PT and aPTT assays. However, this work was conducted utilizing data from the PT and APTT assays, which as discussed earlier, are only sensitive to events associated with the hypocoagulable state. Additionally, the work described was conducted in the presence of strong clot formation because of the addition of supraphysiological concentrations of tissue factor. Fibrin polymerization is significantly altered in a dilute systems designed for global hemostatic assessment resulting in weak and unstable clot formation. Global hemostatic assessment and new methods for monitoring and quantifying fibrin polymerization are required.